The present invention relates to improvements in a photocoupler.
Photocouplers are optical coupling devices which include a semiconductor light emitting element and a semiconductor photo responsive element. They are presently often used for the isolation of solid state relays and transmission lines. Also, a photocoupler array consisting of a plurality of photocouplers mounted on the same substrate or a combined circuit consisting of a photocoupler and an integrated circuit are very advantageous for logic circuits and audio circuits.
As the semiconductor light emitting element for the photocoupler, except in case where a very rapid response is required as for communication, an Si-doped GaAs infrared light emitting diode fabricated by liquid phase epitaxial growth is usually employed for the reasons:
(1) THAT THE LUMINOUS EFFICIENCY IS HIGH, AND
(2) THAT THE EMISSION WAVELENGTH IS APPROXIMATELY 9,400 A to which an Si photo responsive element is highly sensitive.
Usually, the size of the semiconductor light emitting element is approximately 0.3 .times. 0.3 mm.sup.2 or larger for easy handling.
As illustrated in FIG. 2 and FIG. 4 in the official gazette of Japanese Patent Application Publication No. 17862/1967, there are two methods for mounting the semiconductor light emitting element with respect to the semiconductor photo-responsive element.
The first method disposes a P-N junction in the semiconductor light emitting element in parallel with the light receiving face of the semiconductor photo responsive element as shown in FIG. 2 in the official gazette. The photocoupler of such construction is called the principal surface emission type. According to the second method, as shown in FIG. 4 in the official gazette, a P-N junction in the semiconductor light emitting element is disposed perpendicularly to the light receiving face of the semiconductor photo-responsive element. The photocoupler of such an arrangement is termed the side surface emission type.
In the semiconductor light emitting element, the light emission takes place in the vicinity of the P-N junction. Part of the emitted light is absorbed within the crystal of the semiconductor light emitting element, while the remaining part is radiated to the exterior. Accordingly, the brilliance of the radiated light is the highest in the direction of the side surface to which the P-N junction is exposed, and it is comparatively low in the direction of the principal surface under the influence of the internal absorption. In this connection, the brilliance of the light in the direction of the principal surface is about half of that of the light in the direction of the side surface.
In the principal surface emission type photocoupler, the light is radiated from the whole principal surface of the semiconductor light emitting element. Therefore, in case where the light receiving region of the semiconductor photo-responsive element has substantially the same area as that of the principal surface of the semiconductor light emitting element, a major portion of the radiated light reaches the light receiving region and a comparatively high optical coupling efficiency is exhibited. However, when the size of each light receiving region is made small in order to raise the degree of integration of the semiconductor photo responsive elements, a mere fraction of the radiated light reaches the light receiving region, and the remaining light falls on another region of the semiconductor photo-responsive element. As a result, the optical coupling efficiency becomes low. Moreover, the light falling on areas other than the light receiving region becomes stray light and gives rise to malfunctions of the semiconductor photo-responsive element. It has therefore been impossible to make the degree of integration high in a photocoupler array or in a combined device consisting of a photocoupler and an integrated circuit.
On the other hand, in the side surface emission type photocoupler, the width of a light emitting region is as small as about 50 .mu.m, and the light receiving region can be made small accordingly. It is therefore possible to enhance the degree of integration.
Since, however, both the light receiving region and the light emitting region are small, the optical coupling efficiency is not raised satisfactorily unless both the regions are exactly aligned.